This invention relates to a method and apparatus for forming shadow masks of color cathode ray tubes using a particularly configured die or punch press machine.
The shadow mask of a color cathode ray tube (CRT) is mounted within a glass panel and serves to establish correspondence between electron beams emitted from the guns in the neck of the tube and the color phosphor deposited on the inner surface of the CRT screen. The shadow mask has a spherical surface made by draw-forming a planar sheet of starting material, and has a plurality of discrete slots or circular apertures of micron dimensions disposed with precise pitches of sub-micron dimensions and formed by photoetching or the like.
Phosphor of the three basic colors are deposited on the CRT screen behind the shadow mask such that they luminesce when impinged by beams passing through the shadow mask slots or apertures. Precise alignment between the shadow mask openings and the phosphor is essential, and any even slight shift or inaccuracy in such alignment results in color sheer or blur on the CRT screen due to the electron beams impinging the phosphor off-center.
A primary cause of such alignment shift is the thermal deformation of the shadow mask, commonly termed "doming". In recent years, ferro-nickel invar alloys of low thermal expansion have been used as shadow mask materials to prevent such doming.
FIG. 1 shows a conventional die or punch press machine for forming shadow masks, and includes a punch 1 having a convex spherical surface 11, a peripheral groove 121 for forming a loop bead in the shadow mask, and a side surface 13 for forming a skirt around the mask. A blank holder 2 surrounds the punch 1, and has a lower clamping surface 21 defining a groove 211. A die pad 3 is disposed below the punch 1 and has a concave peripheral surface 311 corresponding to the convex surface of the punch and a bead 3111 corresponding to the groove 121. A clamping die 4 surrounds the pad 3 below the blank holder 2, and has an upper clamping surface 41 corresponding to the holder surface 21 and a bead 411 corresponding to the groove 211. The punch 1, the blank holder 2 and the pad 3 are respectively mounted on an inner post 5, outer post(s) 6 and a knockout post 7 of a double-action die press machine. The die 4 rests on a shoulder of the pad 3, and is driven up and down together with the pad.
In operation, a planar sheet of starting material for forming the shadow mask 9 is laid on the die 4 as shown in FIG. 2(a), whereafter the blank holder 2 is forced down upon the die 4 to firmly clamp the periphery of the shadow mask via the mating groove 211 and bead 411. The punch 1 is then lowered as shown in FIG. 2(b) to draw-form a spherical surface 9a of the shadow mask and press-form a loop bead 9b in its periphery. The upper surface of the pad 3 is disposed lower than that of the die 4, which improves the performance of the machine. The punch 1, the pad 3 and the die 4 are then lowered together as shown in FIG. 2(c) with the blank holder 2 remaining stationary, which releases the outer clamping of the shadow mask. After the die 4 reaches its bottom dead center position the downward movement of the punch 1 and the pad 3 is continued as shown in FIG. 2(d), which bends or wipe-forms the skirt 9c of the shadow mask between the outer side 13 of the punch and the inner side 44 of the die. After a short pause in the FIG. 2(d) position to allow elastic transients to settle out, the components are returned to the Fig. 1 position to release the formed shadow mask.
When an invar alloy material is used as the shadow mask in such a draw-forming process, wavy creases or distortions 9d sometimes appear around the edges of the formed mask after its removal due to the "spring back" effect of the high yield strength alloy, as shown in FIG. 3. Such distortions obviously degrade the performance of the CRT, and adversely affect its vibration characteristics.
It is thought that such spring back distortions are attributable to the manufacturing sequence of first forming the spherical surface 9a and loop bead 9b of the shadow mask, and lastly forming its skirt 9c, and in an effort to prevent these distortions and creases when using an invar alloy or the like the conventional press machines have been modified by adding an additional power unit 10a as shown in FIG. 4 for bending the skirt inwardly at a sharper angle, and/or by providing a further power unit 10b for the punch as shown in FIG. 5 to enhance the stiffness of the shadow mask periphery.